1. Field of the Invention
The present invention generally relates to power converters, and, more particularly, to multiple output converters.
2. Description of the Prior Art
Multiple output converters have become increasingly popular as the demand increases for multiple voltage levels within modern electronics. Traditionally, the primary output of a converter is regulated via an error voltage signal fed back to the primary pulse-width modulation (PWM) control circuit, and the secondary outputs are regulated either by linear regulators or magnetic amplifiers.
The use of a linear regulator in a converter circuit is illustrated in FIG. 1. The primary output subcircuit for generating power at output terminal #1 includes a secondary coil 10 that is responsive to a primary coil 21 for generating a voltage/current that passes through a diode 12 and a capacitor 14. A feedback line 16 from output terminal #1 is connected to the primary PWM control circuit 18 through an isolation unit 19. The primary PWM control circuit 18 detects the level of the feedback signal and correspondingly adjusts the fluctuation of the voltage to the control gate of the transistor 17 where the transistor is turned on and off, thereby adjusting the fluctuation of the current going through the primary coil 21. The fluctuation of the current going through the coil 21 causes the primary coil 21 to generate a magnetic field having an intensity that corresponds to the frequency of the current fluctuation. The magnetic field in turn affects the secondary coil 10 and causes the secondary coil 10 to generate a voltage/current at output terminal #1 of the primary output subcircuit (via the diode).
The secondary subcircuit for generating a voltage/current at output terminal #2 comprises a secondary coil 22 responding to the primary coil in generating a voltage/current that passes through a diode 24 and a capacitor 26 where the output at output terminal #2 is regulated by a linear drop regulator 16. While the use of linear regulators are fairly simple, their use is limited to low current outputs because of the relatively low efficiency of the linear regulators. Note that there can be more than one secondary subcircuits for generating different voltages.
For magnetic amplifiers (magamp), referring to FIG. 2, while the primary output subcircuit can be the same, the secondary subcircuit 30 generally comprises a secondary coil 31 for generating a voltage/current that passes through a magamp 32, a diode 34, and a capacitor 36. A magamp control circuit 38 senses the voltage level at output terminal #2 and generates a control signal to node 37 to cause the subcircuit to produce the overall and desired output. While this type of circuit is suitable for high current outputs, it is difficult to control the output when the load current is low. Also, it is a relatively complex circuit when compared to a semiconductor switch.
Recently, a third option has become available with the introduction of secondary side post regulator (SSPR) control circuits, which can be in the form of application-specific integrated circuits (ASICs). Referring to FIG. 3, a secondary converter subcircuit comprises a secondary coil 42 for generating a voltage/current that passes through a diode 44, transistor M1, and a capacitor 48. A SSPR ASIC 50, sensing the voltage/current levels between two points (output terminal #2 and the sync node), generates a control signal to the gate terminal of a transistor M1 to control the voltage level generated at output terminal #2. The SSPR circuit has a simpler design than that of the magamp and provides better efficiency than that of a linear regulator. The use of SSPR PWM ASIC can significantly improve converter efficiency and simplify the converter design process. On the other hand, a converter using a SSPR/MOSFET combination is no more efficient than a converter using a magamp. In fact, it can be argued that a magamp is a more efficient switch than a SSPR/MOSFET switch.
However, all three types of circuits utilize a diode for the secondary subcircuits on the secondary side, which reduces converter efficiency--particularly for low voltage and high current output applications. These conventional flyback converters require a diode in the subcircuits as shown in FIGS. 1-3 to prevent the capacitor from discharging when the primary switch is turned on. By using a diode in the circuit path, it results in a voltage drop of approximately 0.6 volt.
It would be desirable to have a converter circuit that does not utilize a component such as a diode so that there is not a voltage drop in the converter circuit.